Self-Assembly of Fluorinated Boronic Esters and 4,4′-Bipyridine into 2:1 N→B Adducts and Inclusion of Aromatic Guest Molecules in the Solid State: Application for the Separation of o, m, p -Xylene

Article abstract of DOI:10.1021/acs.cgd.7b01368

A series of 2:1 fluorinated arylboronic ester adducts with 4,4′-bipyridine sustained by N→B dative bonds have been synthesized. The degree of fluorination in the arylboronic esters derived from catechol is shown to modulate the molecular conformation of t

Full text of DOI:10.1021/acs.cgd.7b01368

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Self-Assembly of Fluorinated Boronic Esters and 4,4’-Bipyridine
into 2:1 N#B Adducts and Inclusion of Aromatic Guest Molecules
in the Solid-State - Application for the Separation of o,m,p-Xylene
Gonzalo Campillo-Alvarado, Eva Cecilia Vargas-Olvera, Herbert Höpfl, Angel
D. Herrera-España, Obdulia Sánchez-Guadarrama, Hugo Morales-Rojas,
Leonard R. MacGillivray, Braulio Rodríguez-Molina, and Norberto Farfan
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01368 • Publication Date (Web): 03 Apr 2018
Downloaded from http://pubs.acs.org on April 3, 2018
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SelfꢀAssembly of Fluorinated Boronic Esters and 4,4’ꢀBipyridine into 2:1
N→B Adducts and Inclusion of Aromatic Guest Molecules in the Solidꢀ
State − Application for the Separation of o,m,pꢀXylene
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Gonzalo CampilloꢀAlvarado,a,b,† Eva C. VargasꢀOlvera,a,† Herbert Höpfl,*,a Angel D.
HerreraꢀEspaña,a Obdulia SánchezꢀGuadarrama,a Hugo MoralesꢀRojas,*,a Leonard R.
MacGillivray,b Braulio RodríguezꢀMolina,c Norberto Farfánd
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[a]
Centro de Investigaciones Químicas, Instituto de Investigación en Ciencias Básicas y
Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001,
Chamilpa, Cuernavaca 62209, Morelos, México.
Eꢀmail: hhopfl@uaem.mx, hugom@uaem.mx (authors to whom correspondence should
be addressed). Tel., Fax: +52 777 329 7997
Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA.
Instituto de Química, Universidad Nacional Autónoma de México, Ciudad de México
04510, México.
Facultad de Química, Departamento de Química Orgánica, Universidad Nacional
Autónoma de México, Ciudad de México 04510, México.
[b]
[c]
[d]
Abstract
A series of 2:1 fluorinated arylboronic ester adducts with 4,4’ꢀbipyridine sustained by
N→B dative bonds has been synthesized. The degree of fluorination in the arylboronic
esters derived from catechol is shown to modulate the molecular conformation of the
coordinated boronic ester moieties and the intermolecular interactions by means of C−H⋅⋅⋅F
and F⋅⋅⋅F contacts that sustain the crystal lattices. The adduct derived from the catechol
ester of 2,4ꢀdifluorophenylboronic acid was chosen to examine the formation of inclusion
complexes with a large number of aromatic guests, affording solvates, cocrystals and a
cocrystal solvate. Six different crystal structure types with 1:1, 1:2 and 1:2:2 N→B
adduct−guest ratios were observed, whose supramolecular organization is strongly
influenced by the formation of sandwichꢀtype complexes between the host and guest
molecules. The hostꢀguest interactions involve π⋅⋅⋅π interactions with the bipyridine linkers
and additional contacts with the catecholate and BꢀarylF substituents, indicating a large
flexibility of the N→B adducts to adapt to the guest stereochemistry. The versatility of the
crystallization system was employed to isolate oꢀxylene from an equimolar mixture of o−,
m− and p−xylene.
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Keywords: Molecular crystals; boron adducts; aromatic guest inclusion; π⋅⋅⋅π, C−H⋅⋅⋅F and
F⋅⋅⋅F contacts; xylene isomer separation
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1. Introduction
The rational design of supramolecular architectures based on the selfꢀassembly of building
blocks (tectons)1,2 has led to the generation of diverse functional molecular systems, such as
macrocycles, cages, capsules, rotaxanes, calixarenes, and MOFs.3ꢀ6 The assemblies are
frequently metalꢀorganic constructs with coordinate covalent bonds being the essential
element of junction between the tectons. More recently, supramolecular structures with a
high level of organization based on weak nonꢀcovalent intermolecular interactions have
become increasingly valuable with applications in hostꢀguest chemistry7,8 and crystal
engineering.9 In the context of ion and molecular recognition, much attention has been
directed towards the sensing and/or removal of toxic chemicals, including small aromatic
hydrocarbons10,11 and polycyclic aromatic hydrocarbons (PAHs),12,13 for which most of the
current techniques rely on costly processes.14
Easily accessible boronic acids have been a useful tool for the rational design of functional
materials due to the capability of forming hydrogenꢀbonded homoꢀ and heterodimeric
synthons15,16 as well as aggregates with reversible covalent B–O bonds.17ꢀ19 Among others,
boronic acids have been employed to assemble discrete and infinite systems capable of
recognizing anions (i.e. Fꢀ, CNꢀ) and neutral molecules including aromatic compounds.20ꢀ24
Moreover, in the presence of Nꢀcontaining ligands, the formation of boronꢀbased
supramolecular aggregates through coordinate covalent N→B bonds gives access to more
diversified systems including coordination polymers, nanostructures and gels.25ꢀ27 Recent
examples are 2:1 Lewis type N→B adducts formed from boronic esters and diamines, of
which the former are derived from arylboronic acids and catechol derivatives. When using
dipyridyl moieties as connectors, upon coordination to boron the nitrogen atoms and the π–
cloud become electronꢀdeficient and, thus, suitable for the recognition and selective
isolation of aromatic hydrocarbons.24 As illustrated in Scheme 1, these adducts have two
sites for guest recognition providing them with a doubleꢀtweezer or Hꢀtype structure.
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Although in solution generally dissociation of N→B bonds does occur even in the presence
of lowꢀpolar solvents,26,28,29 our preliminary studies have shown that there might be
interesting applications for the selective recognition and inclusion of PAHs within the solid
state.24 The following general characteristics are visualized for such boronꢀamine adducts,
which are relevant for a widespread range of applications: (i) The assembly of the 2:1
adducts can be achieved in singleꢀstep crystallizationꢀinduced multicomponent reactions.
(ii) The electronic and steric properties of the molecular framework can be easily modified
either by variation of the arylꢀsubstituent at the boron atoms or the catechol moieties, since
a large variety of these reagents are commercially available (Scheme 1a). Among other
properties, this allows for the creation of additional sites for the recognition of specific
molecules and, considering the structural syn/antiꢀisomerism of the host structures, the
simultaneous recognition of two different aromatic guest molecules might be envisioned
(Scheme 1b). (iii) The size of the diamine spacer and its electronic properties can be varied
using either aliphatic, aromatic or combined aliphaticꢀaromatic connectors. (iv) More
complex systems with confined cavities can be achieved by enhancing the nuclearity of the
adduct, e.g. by using triꢀ or tetraꢀamine ligands. These perspectives illustrate that this class
of molecules could constitute an interesting alternative to Stoddart’s Xꢀboxes and –
cages,30,31 Peinador’s and Fujita’s palladium and platinumꢀbased metallacycles and –
cages,32,33 Severin’s and Iwasawa’s oligonuclear boron cages,23,34 among others,35,36 all of
which have shown high selectivity for the inclusion of large PAHs.
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Scheme 1. a) Illustration of some relevant characteristics of doubleꢀtweezer adducts based on N→B
interactions concerning the recognition of aromatic hydrocarbons. b) Possible syn/antiꢀstereoisomers of the
2:1 adducts, of which the synꢀisomer might be suitable for the recognition of two different guest molecules.
In our previous communication, we reported on the formation of doubleꢀtweezer adducts of
the boronic ester formed from phenylboronic acid and 2,3ꢀdihydroxynaphthalene in
combination with 1,2ꢀbis(4ꢀpyridyl)ethylene, 1,2ꢀbis(4ꢀpyridyl)ethane and 4,4’ꢀazopyridine
as diamine linkers, which enabled the recognition of naphthalene, fluorene, phenanthrene,
anthracene, and pyrene via the formation of solid inclusion compounds.24 In continuation of
this ongoing research project, we report herein on the selfꢀassembly, inclusion chemistry
and application (viz. the separation of oꢀxylene from a mixture of oꢀ, mꢀ and pꢀxylene) of
solids based on 2:1 adducts prepared from fluorinated arylboronic esters and 4,4´ꢀ
bipyridine (bpy). The presence of fluorine atoms in the boronic esters derived from
catechol is expected to enhance the Lewis acidity of the boron atoms and, thus, to
strengthen the N→B bond.37,38 As a consequence, the increased ionic character of the N→B
bond should increment the electron deficiency in the dipyridyl spacer, which constitutes the
central site for the recognition of aromatic hydrocarbons by means of π⋅⋅⋅π interactions.24
Furthermore, using different fluorinated boronic esters, the variability of intermolecular
interactions such as C−H⋅⋅⋅X (X = N, O, F) and C−H⋅⋅⋅π contacts might in turn facilitate the
formation of either solventꢀfree tweezers, solvates or cocrystals with suitable guests,
enabling selective molecular recognition.
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2. Experimental section
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Instrumental. Elemental analyses were performed on crystalline samples by using Fisons
EAꢀ1108 and Elementar Vario ELIII element analyzers. It should be noted that boron
compounds produce incombustible residues that complicate elemental analyses in terms of
the established limits of exactitude.39 IR spectra were recorded on a FTꢀIR Nicolet 6700
ThermoScientific spectrophotometer and measured in the range of 4000ꢀ400 cmꢀ1 using the
Smart iTR accessory with a diamond ATR crystal. Thermogravimetric and differential
scanning calorimetry (TGꢀDSC) analyses were performed on a TA SDT Q600 instrument
with approximately 3 mg of a solid sample at a heating rate of 10 °C/min within the
temperature range of 30ꢀ500 °C using a current of nitrogen as inert gas purge (50 mL/min).
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Preparative Part. The boronic esters (i.e. catechol (cat) esters of phenylboronic (phbe), 4ꢀ
fluorophenylboronic
(4fphbe),
2,4ꢀdifluorophenylboronic
(24fphbe),
2,4,6ꢀ
trifluorophenylboronic (246fphbe) and pentafluorophenylboronic (pfphbe) acid) were
prepared according to the literature using catechol and the corresponding arylboronic acid29
and the characterization of products was in agreement with a previous report.37 All starting
materials and solvents were commercially available from SigmaꢀAldrich and were used as
received without further purification. The preparative methods for the formation of the
N→B adducts and the inclusion complexes were carried out under ambient atmosphere.
Liquidꢀassisted grinding (LAG) experiments were performed using mortar and pestle or a
Retsch MM400 mixer mill operated at 25 Hz and equipped with stainless steel grinding jars
(1.5 mL). Before starting, 1 to 3 drops of solvent were added to approximately 40 mg of a
mixture corresponding to a 2:1 stoichiometric ratio of the boronic ester and bpy. The
resulting powder was then distributed on a filter paper and airꢀdried.
Preparation of [(C6H5)BO2C6H4]2[bpy] (A1). A mixture of phbe (0.015 g, 0.076 mmol)
and bpy (0.006 g, 0.038 mmol) was dissolved in 3.5 mL of ethyl acetate. The solution was
gently heated in a sealed vial until all the starting material had dissolved and the solution
became clear. After three days of slow evaporation, orange crystals suitable for Xꢀray
diffraction analysis were obtained and collected by filtration. Yield: 0.016 g (78%). M.p.
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168−171 °C. B NMR (CDCl3, 64 MHz): δ 27 (h1/2 = 39 Hz) ppm. IR (ATR): ṽ = 3026
(w), 1623 (m), 1596 (m), 1541 (w), 1507 (w), 1476 (s), 1439 (w), 1417 (m), 1366 (w),
1326 (w), 1237 (m), 1223 (m), 1207 (s), 1148 (m), 1099 (m), 1079 (m), 987 (m), 949 (m),
865 (m), 823 (m), 804 (m), 755 (m), 740 (m), 713 (s), 685 (s), 652 (m), 628 (m). Elemental
analysis (%) for C34H26B2N2O4 (548.21) Calcd: C, 74.49; H, 4.78; N, 5.11. Found: C,
73.20; H, 4.64; N, 4.87.
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Preparation of [(4ꢀFC6H4)BO2C6H4]2[bpy] (A2). A mixture of 4fphbe (0.014 g, 0.065
mmol) and bpy (0.005 g, 0.032 mmol) was dissolved in 3.5 mL of benzene, whereupon the
procedure used for A1 was followed to afford orange crystals. Yield: 0.014 g (75%). M.p.
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185−190 °C. B NMR (CDCl3, 64 MHz): δ 30 (h1/2 = 132 Hz) ppm. IR (ATR): ṽ = 3039
(w), 1629 (m), 1590 (m), 1541 (w), 1507 (w), 1473 (s), 1416 (m), 1359 (w), 1232 (m),
1200 (s), 1152 (m), 1084 (m), 982 (m), 956 (m), 811 (m), 760 (m), 726 (s), 706 (m), 581
(w). Elemental analysis (%) for C34H24B2F2N2O4 (584.19): Calcd.: C, 69.90; H, 4.14; N,
4.80. Found: C, 70.28; H, 3.67; N, 5.04.
Preparation of [(2,4ꢀF2C6H3)BO2C6H4]2[bpy] (A3). A mixture of 24fphbe (0.015 g, 0.064
mmol) and bpy (0.005 g, 0.032 mmol) was dissolved in 3.5 mL of acetonitrile, whereupon
the procedure used for A1 was followed to afford pale yellow crystals that were collected
by filtration. Yield: 0.011 g (55 %). M.p. 192ꢀ196 °C. 11B NMR (CDCl3, 64 MHz): δ 28
(h1/2 = 76 Hz), 20 (h1/2 = 132 Hz) ppm. IR (ATR): ṽ = 3063 (w), 2357 (w), 1621 (m), 1604
(m), 1473 (s), 1413 (m), 1407 (w), 1232 (s), 1200 (s), 1129 (m), 1090 (m), 1078 (m), 993
(s), 950 (m), 857 (m), 828 (m), 774 (s), 743 (s), 689 (m), 584 (m). Elemental analysis (%)
for A3, C34H22B2F4N2O4 (620.17): Calcd.: C, 65.85; H, 3.58; N, 4.52. Found: C, 65.63; H,
3.56; N, 4.40.
Preparation of [(2,4,6ꢀF3C6H2)BO2C6H4]2[bpy] (A4). A mixture of 246fphbe (0.015 g,
0.060 mmol) and bpy (0.005 g, 0.030 mmol) was dissolved in 3.5 mL of chloroform and
the same procedure used for A1 was followed to afford pale yellow crystals that were
collected by filtration. Yield: 0.016 g (81 %). M.p. 212ꢀ216 °C. 11B NMR (CDCl3, 64
MHz): δ 27 (h1/2 = 25 Hz), 19 (h1/2 = 38 Hz) ppm. IR (ATR): ṽ = 3069 (w), 1628 (m), 1606
(m), 1473 (s), 1590 (m), 1476 (s), 1419 (m), 1235 (s), 1207 (s), 1188 (m), 1163 (m), 1109
(s), 1079 (s), 1000 (s), 968 (m), 916 (m), 847 (m), 827 (m), 769 (m), 746 (m), 736 (s), 699
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(m), 579 (m), 540 (m), 525 (m). Elemental analysis (%) for C34H20B2F6N2O4 (656.15)
Calcd.: C, 62.24; H, 3.07; N, 4.27. Found: C, 62.37; H, 2.94; N, 4.31.
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Preparation of [(2,3,4,5,6ꢀF5C6)BO2C6H4]2[bpy] (A5). A mixture of pfphbe (0.016 g,
0.076 mmol), catechol (0.008 g, 0.073 mmol) and bpy (0.006 g, 0.038 mmol) was
dissolved in 25 mL of benzene and the solution was heated under reflux with the use of a
Dean−Stark trap for 3 h, whereupon the resulting solution was concentrated (ca. 5 mL).
Cooling the reaction mixture slowly to room temperature afforded orange crystals of A5.
Yield: 0.009 g (32%). M.p: 200−205 °C. 11B NMR (CDCl3, 64 MHz): δ 27 (h1/2 = 101 Hz),
12 (h1/2 = 101 Hz) ppm. IR (ATR): ṽ = 2970 (w), 1646 (m), 1516 (w), 1470 (s), 1416 (m),
1391 (w), 1291 (s), 1229 (m), 1138 (m), 1084 (m), 1036 (m), 962 (s), 913 (w), 845 (s), 794
(m), 734 (s), 695 (s), 607 (m), 552 (m). Elemental analysis (%) for C34H16B2F10N2O4
(728.12): Calcd.: C, 56.09; H, 2.21; N, 3.85. Found: C, 56.42; H, 1.99; N, 3.66.
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General procedure for the synthesis of solvates with A3
A mixture of 24fphbe (0.015 g, 0.064 mmol) and bpy (0.005 g, 0.032 mmol) was dissolved
in 3.5 mL of an aromatic solvent (i.e. benzene, toluene, oꢀxylene, mꢀxylene, pꢀxylene,
mesitylene or benzonitrile). The solution was gently heated in a sealed vial until the starting
materials were completely dissolved. Crystals formed upon slow evaporation of the
solution at room temperature.
General procedure for the synthesis of cocrystals with A3
A mixture of 24fphbe (0.015 g, 0.065 mmol) and bpy (0.005 g, 0.033 mmol) in 3.5 mL of
acetonitrile was added to a vial containing a solution of the appropriate solid guest (i.e.
naphthalene, naphthol, 3ꢀbromophenol or aniline) in 1.0 mL of acetonitrile. The solution
was gently heated in a sealed vial until the starting materials were completely dissolved.
Crystals formed upon slow evaporation of the solution at room temperature. For the case of
A3⊃TTF⋅MES, mesitylene was used as crystallization solvent and the same procedure was
followed.
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Xꢀray diffraction analyses
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Powder Xꢀray diffraction (PXRD) analyses were carried out in the transmission mode on a
BRUKER D8ꢀADVANCE diffractometer equipped with a LynxEye detector (λCuꢀKα1
1.5406 Å, monochromator: germanium). The equipment was operated at 40 kV and 40 mA,
and data were collected at room temperature in the range of 2 = 5ꢀ60º. Single–crystal Xꢀ
=
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θ
ray diffraction (SCXRD) studies for A1, A2 and A5 were performed on a BrukerꢀAPEX
diffractometer with a CCD area detector (λΜοΚα = 0.71073 Å, monochromator: graphite).
Frames were collected at T = 100 for A1 and T = 293 K for A2 and A5 via ω/φꢀrotation at
10 s per frame (SMART).40 The measured intensities were reduced to F2 and corrected for
absorption with SADABS (SAINTꢀNT).41 Corrections were made for Lorentz and
polarization effects. Structure solution, refinement and data output were carried out with the
SHELXTLꢀNT program package.42,43 Non hydrogen atoms were refined anisotropically,
while hydrogen atoms were placed in geometrically calculated positions using a riding
model. Data for the remaining compounds were collected on an Agilent Technologies
SuperNova diffractometer equipped with a CCD area detector (EosS2) using CuꢀKα
radiation (λ = 1.54184 Å) for A3⊃ANI and A3⊃NAPOH, and MoꢀKα radiation (λ=
0.71073 Å) for A3⊃BEN, A3⊃TOL, A3⊃oXYL, A3⊃BENCN, A3⊃BrPOH and
A3⊃TTF⋅MES. Frames were collected at T = 100 K in all cases with exception of
A3⊃BENCN, for which the crystals developed fissures at this temperature. In this case
data were acquired at T = 200 K. The measured intensities were reduced to F2 and corrected
for absorption using spherical harmonics (CryAlisPro).44 Intensities were corrected for
Lorentz and polarization effects. Structure solution, refinement, and data output were
performed with the OLEX245 program package using SHELXLꢀ201446 for the refinement.
Nonꢀhydrogen atoms were refined anisotropically. All hydrogen atoms were placed in
geometrically calculated positions using the riding model.
In all crystal structures except for A3 and A4, the molecular structures of the N→B adducts
have crystallographic inversion symmetry, with the asymmetric unit consisting of one
molecule half. The only exception is A3⊃BENCN, which comprises two
crystallographically independent molecule halves. In the crystal structures of A3⊃BEN,
A3⊃TOL, A3⊃ANI, and A3⊃oXYL, the Bꢀaryl rings exhibit disorder over two positions
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providing two different orientations of the orthoꢀfluorine atoms (A3⊃BEN, occ = 0.77 and
0.23; A3⊃TOL, occ = 0.59 and 0.41; A3⊃ANI, occ = 0.67 and 0.33; A3⊃oXYL, occ =
0.50 and 0.50). In addition, in A3⊃TOL the bpy linkers are disordered over two positions,
which were refined with occupancy factors of 0.56 and 0.44. In A3⊃TOL, A3⊃ANI and
A3⊃oXYL the aromatic guest molecules are disordered over two positions (A3⊃TOL, occ
= 0.50 and 0.50; A3⊃ANI, occ = 0.50 and 0.50; A3⊃oXYL, occ = 0.68 and 0.32). For
A3⊃TOL, the disorder was refined using Uij restraints (SIMU, EADP) implemented in
SHELXLꢀ2014.46 Diamond and Olex2 were used for the creation of figures.45,47
Crystallographic data for the structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publications no. CCDCꢀ
1574794−1574806. Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223ꢀ336ꢀ033; eꢀmail:
deposit@ccdc.cam.ac.uk, www: http://www.ccdc.cam.ac.uk).
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3. Results and Discussion
3.1. Synthesis of N→B adducts A1−A5
N→B adducts A1−A4 were obtained in two reaction steps. First, the boronic esters were
prepared following a previously reported procedure for esterification using catechol and the
corresponding arylboronic acid as the starting materials:29 A1, phenylboronic acid; A2, 4ꢀ
fluorophenylboronic acid; A3, 2,4ꢀdifluorophenylboronic acid;
A4, 2,4,6ꢀ
trifluorophenylboronic acid. Then, the 2:1 tweezer type compounds were synthesized by
gently heating a mixture of the boronic ester with 4,4’ꢀbipyridine (bpy) either in ethyl
acetate (A1), acetonitrile (A2), acetonitrile (A3) or chloroform (A4), followed by slow
evaporation of the solvent to afford crystals of orange (A1, A2) or yellow (A3, A4)
coloration. Because of the low stability of the boronic ester derived from 2,3,4,5,6ꢀ
pentafluorophenylboronic acid against hydrolysis, in this case the adduct (A5) was
synthesized by a singleꢀstep multicomponent 2:2:1 condensation reaction in benzene,
starting from the respective boronic acid, catechol and bpy (Scheme 2). Compounds
A1−A5 were characterized by elemental analysis, IR spectroscopy (Fig. S1−S5, Supporting
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Information) and singleꢀcrystal Xꢀray diffraction analysis. Phase purity of A1−A5 was
evidenced by powder Xꢀray diffraction (PXRD) analysis, which allowed for a comparison
with the patterns simulated from the singleꢀcrystal Xꢀray diffraction (SCXRD) studies (Fig.
S6ꢀS10, Supporting Information).
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Scheme 2. Synthesis of the 2:1 N→B adducts A1ꢀA5 via crystallization and liquidꢀassisted grinding methods.
Alternatively, the arylboronic ester−bpy adducts A1−A5 could be synthesized using a solid
state reaction protocol, i.e. liquidꢀassisted grinding (LAG),48 which was carried out for 30
min in a mortar after adding 1 to 3 drops of solvent (Table 1). Product formation becomes
evident immediately following the color change from colorless to yellow or orange upon
adduct formation (Fig. S11, Supporting Information). With the purpose to reduce the
grinding time, a selected sample (A2) was prepared also using a commercial mixer mill,
showing that the product can be achieved within 5 minutes either from benzene or
acetonitrile (Figure 1).
Solidꢀstate synthesis by grinding techniques is increasingly employed in crystal
engineering, particularly in the field of cocrystals with active pharmaceutical ingredients.49
More recently, these methodologies have been extended to coordination polymers including
MOFs.50,51 For the solidꢀstate formation of boronic esters, only a few research groups have
established protocols52ꢀ54 and, to the best of our knowledge, there is only one previous
report on the mechanochemical generation of N→B adducts.55
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Table 1. Methods employed for the preparation of adducts A1−A5.
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6
Method1
Adduct
Bꢀaryl substituent
7
8
9
phenyl
Crystallization, LAG
Crystallization, LAG
Crystallization, LAG
Crystallization, LAG
A1
A2
A3
A4
A5
4ꢀfluorophenyl
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2,4ꢀdifluorophenyl
2,4,6ꢀtrifluorophenyl
2,3,4,5,6ꢀpentafluorophenyl Crystallization, LAG2
1
Crystallization or solution method: Singleꢀcrystals were obtained
after slow evaporation of a briefly heated solution of the
components in 2:1 stoichiometric boronic ester:bpy ratio in ethyl
acetate (A1), acetonitrile (A2), acetonitrile (A3), and chloroform
(A4). Liquidꢀassisted grinding (LAG): The components were
ground for 20 Min in a mortar after addition of a drop of the
solvent used in the crystallization method.
Adduct A5 was prepared in benzene via
multicomponent reaction starting from pentafluorophenylboronic
acid, catechol and bpy or by LAG (benzene).
2
a singleꢀstep
Figure 1. PXRD patterns of the liquidꢀassisted grinding experiments for the formation of A2 using a
commercial mixer mill (25 Hz, 5 min) and either acetonitrile or benzene as solvent. For comparison, the
PXRD patterns of the starting materials (4fphbe and bpy) and the pattern of A2 simulated from the SCXRD
data are also included.
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3.2. Crystallographic analysis of adducts A1−A5
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The crystalline substances obtained for adducts A1−A5 contained crystals suitable for
singleꢀcrystal Xꢀray diffraction (SCXRD) analysis. The most relevant crystallographic data
for these adducts are summarized in Table 2.
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Table 2. Selected crystallographic data for adducts A1, A2, A3, A4 and A5.
A1a
A2a
A3b
A4b
A5a
Formula
MW (g molꢀ1)
T (K)
C34H26B2N2O4 C34H24B2F2N2O4 C34H22B2F4N2O4 C34H20B2F6N2O4 C34H16B2F10N2O4
548.19
100
584.17
293
620.15
100
656.14
100
728.11
293
Space group
Radiation
a (Å)
P21/c
P21/c
P21/c
P21/c
P21/n
Mo Kα
8.0059(9)
14.3024(16)
12.4637(14)
90
Mo Kα
8.282(2)
14.290(4)
12.317(3)
90
Cu Kα
13.3229(2)
13.0653(2)
18.2981(4)
90
Cu Kα
Mo Kα
9.128(4)
13.476(6)
12.331(5)
90
13.16759(14)
13.37769(12)
18.0080(2)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
103.040(2)
90
100.518(5)
90
110.575(2)
90
109.9535(12)
90
92.874(8)
90
1390.3(3)
2
1433.2(7)
2
2981.94(11)
4
2981.72(6)
4
1515.1(11)
2
Z
Z’
0.5
0.5
1
1
0.5
µ (mmꢀ1)
0.085
0.096
0.900
1.033
0.145
ρ
calcd (g cmꢀ3)
1.309
1.354
1.381
1.462
1.596
R1 (Fo > 4σFo)
wR2 (all data)
GOF
0.0380
0.0920
1.043
0.0403
0.1126
1.027
0.0512
0.1471
1.047
0.0388
0.1036
1.061
0.0975
0.1716
1.179
CCDC
1574794
1574795
1574797
1574806
1574804
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3.2.1. Molecular structure analysis of A1−A5
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The crystallographic analysis of A1 revealed the expected molecular structure, in which
two phenylboronic ester molecules are linked by N→B bonds to bpy. The adduct
molecules exhibit crystallographic inversion symmetry located at the central Cpy−Cpy bond
of the ligand with antiparallel orientation of the boronic ester functions (Fig. 2). Table 3
summarizes some representative geometric parameters. The geometry of the boron atom
with a N→B bond length of 1.652(2) Å and a tetrahedral character (THC) of 73.4 % is
comparable to structurally related monoꢀadducts of boronic esters with pyridine
derivatives.29 The molecular extension along the N⋅⋅⋅N axis can be described by the
intramolecular B⋅⋅⋅B separation of 10.35 Å and the centroid⋅⋅⋅centroid distance of 12.26 Å
between opposite catecholate and Bꢀaryl rings. The extension of the lateral boronic esters is
approximately 10.25 Å without considering the van der Waals radii of the terminal
hydrogen atoms. The molecular conformation can be described by the following general
parameters: (i) the angles αpy, αcat and αaryl formed between the mean planes of the
aromatic rings present in the adduct, which according to Severin’s suggestions are reported
in relation to a reference plane defined by the N, B and Ci atoms.56; (ii) the angles βpy,cat
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β
py,aryl and βcat,aryl, which are derived from the intersection of straight lines formed between
the boron atom and the centroids of the respective aromatic rings; and, (iii) the torsion
angle centroid(cat)−B−B−centroid(aryl) evaluating the existence of an overall molecular
twist.
As seen from Fig. 2 and Table 3, the mean planes of the aromatic substituents of the
boronic ester (labelled aryl and cat) and the bpy linker (labelled py) are significantly
twisted between each other (αpy = 86.5º; αcat = 89.6º; αaryl = 37.3º).57ꢀ60 The value of αaryl
indicates that part of the C−H hydrogens of the Bꢀaryl ring are pointing towards the cavity.
An orthogonal orientation to the NBCi reference plane would give a value of 90° similar to
that found for αcat and αpy, while a complete inward rotation would give a value of 0º.
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Table 3. Selected geometric parameters of molecular structures of A1, A2, A3, A4 and A5.
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5
6
A1
A2
A3
A4
A5
7
8
N→B (Å)
1.652(2)
1.654(2)
1.664(2)
1.660(3)
1.666(2)
1.671(2)
1.655(8)
9
1.471(2)
1.478(2)
1.463(2)
1.464(2)
1.470(2)
1.468(2)
1.468(2)
1.467(2)
1.460(2)
1.468(2)
1.469(2)
1.458(2)
1.462(7)
1.453(7)
B−O (Å)
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1.597(2)
106.4(1)
73.4
1.587(2)
106.6(1)
71.9
1.595(3)
1.598(3)
1.611(2)
1.605(2)
1.616(8)
107.2(4)
74.1
B−C (Å)
106.9(1)
107.2(1)
107.0(1)
107.3(1)
O−B−O (°)
Boron THC (%)
70.7
69.5
67.7
68.9
10.35
12.26
10.32
12.18
10.37
10.39
10.36
12.20
B⋅⋅⋅B (Å)
arylB⋅⋅⋅cat (Å)[a]
11.81
11.98
11.94
11.74
Angles between mean planes (º)[b]
py,py (º)
0.0
0.0
0.0
0.0
11.4
1.2
10.6
1.4
0.0
0.0
cat,cat (º)
aryl,aryl (º)
0.0
0.0
5.1
5.6
0.0
86.5
80.3
74.9
79.7
83.7
77.9
87.5
αpy (º)
αcat (º)
αaryl (º)
89.6
37.3
89.4
40.5
89.8
89.0
89.7
89.1
88.5
89.9
58.6
60.2
66.6
60.9
105.6
109.6
144.8
105.2
109.1
145.7
104.8
104.9
103.4
103.5
105.2
108.7
146.1
β
β
β
py,cat (º)
py,aryl (º)
cat,aryl (º)
103.4
104.2
102.4
103.4
151.2
152.6
154.2
153.1
Torsion angles along B⋅⋅⋅B axis (º)[c]
aryl−B1−B1’/B2−aryl
180.0
ꢀ1.0
180.0
ꢀ0.2
ꢀ173.9
175.4
180.0
ꢀ1.7
cat−B1−B1’/B2− aryl
12.6
1.6
ꢀ3.9
ꢀ6.2
cat−B1−B1’/B2−cat
180.0
180.0
ꢀ171.9
174.5
180.0
[a] Distance between the centroids of opposite Bꢀaryl substituents (arylB) and catecholate (cat) rings along the long N⋅⋅⋅N molecular axis.
[b] The mean planes refer to the individual pyridyl rings (py) within bpy, the aromatic ring of the catecholate fragment (cat) and the
aromatic ring of the Bꢀaryl substituent (aryl). αpy, αcat and αaryl are the angles formed between the mean planes of the aromatic rings in the
adduct and, according to Severin’s suggestions, a reference plane defined by the N, B and Ci atoms.56 βpy,cat
βpy,aryl and βcat,aryl are the angles
at the intersection of straight lines formed between the boron atom and the centroids of the respective aromatic rings. [c] Aryl and cat
represent the centroids of the catecholate and Bꢀaryl substituents, respectively. B1 and B1’/B2 correspond to the boron atoms in the
molecular structure.
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Figure 2. Perspective view of the molecular structure of A1. Ellipsoids are drawn at the 50% probability level.
The molecular structure of the N→B adduct prepared from the 4ꢀfluorophenylboronic ester
analogue (A2) exhibits similar structural characteristics (Table 3). The same is true in
almost all aspects for compounds A3−A5, an important exception being the twist of the Bꢀ
aryl rings that carry orthoꢀfluorine atoms (αaryl = 37.3−89.9° for A1ꢀA5, Table 3 and Fig.
S12). The variations of the twist are also illustrated by the torsion angles given in Fig. 3a,
showing only a small change (less than 3°) between A1 and A2, but spanning a
significantly larger range for A3−A5. The significant differences in the twists of αaryl for
A4 and A5, both of which carry fluorine atoms in the 2,6ꢀpositions indicate that the
changes are not only due to intramolecular steric hindrance effects, but also to variations in
the intermolecular connectivity based on C−H⋅⋅⋅F hydrogen bonding interactions (vide
infra). On the contrary, the (cat)B(py) segments of the molecular structures in A1−A5
exhibit only relatively small variations for the angles of the cat and py mean planes with
the NBCi reference plane (αpy 74.9−87.5°; αcat 88.5−89.8°; Table 3, Fig. 3b).61
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Figure 3. a) Comparison of the conformation of the Bꢀaryl substituents in the molecular structures of A1−A5
in relation to the B(cat)(py) segment by means of selected torsion angles. b) Overlay of the molecular
fragments given in (a), indicating a flexible orientation of the Bꢀaryl groups with fluorine atoms in orthoꢀ
position, thus facilitating the formation of stabilizing intermolecular C−H···F contacts. Note: For the
generation of Fig. 3a, the atom numbering was changed in relation to the atom labelling used in the SCXRD
structures in order to allow for a uniform and consistent presentation of the values for the torsion angles. For
A3 and A4, which are not located at crystallographic inversion centers, the data for the second molecule half
are: A3: 52.8°, ꢀ132.8°; A4: 68.1°, ꢀ116.9°.
3.2.2. Supramolecular structure analysis for A1−A5
From a crystal engineering point of view, the molecular structures of A1−A5 are
susceptible to a series of intermolecular interactions including π⋅⋅⋅π, C−H⋅⋅⋅O and C−H⋅⋅⋅π
contacts and, for the fluorine derivatives A2−A5, additionally C−H⋅⋅⋅F and F⋅⋅⋅F contacts.62ꢀ
65
It can be expected that the presence of electronꢀdeficient (bpy coordinated to boron
atoms) and electronꢀrich aromatic substituents (cat and Bꢀaryl) facilitate the formation of
strong π⋅⋅⋅π interactions, which will likely dominate the supramolecular solidꢀstate
organization. The importance of π⋅⋅⋅π interactions for the structural organization of
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molecular crystals and MOFs is a topic of current interest in crystal engineering.15,66ꢀ69 In
this context, the crystal structure analysis of the fluorineꢀfree boronic ester adduct A1
reveals the presence of π⋅⋅⋅π contacts involving the N→B coordinated bpy groups and the
Bꢀphenyl substituents of two neighboring adduct molecules with centroid⋅⋅⋅centroid
distances of 3.87 Å. The formation of π⋅⋅⋅π contacts between the electronꢀdeficient bpy
linkers and the Bꢀphenyl group and not the catecholate moieties is somewhat surprising,
because DFT calculations on catechol esters of fluorinated arylboronic acids at the
RB3LYP/6ꢀ311G(2df,p) level have shown that the HOMO orbitals appear over the boron
atom and the Bꢀcatecholate fragment.62
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As seen from Fig. 4a, in A1 each bpy linker is sandwiched by two Bꢀphenyl groups giving
rise to a triad, which by means of the Bꢀphenyl groups at the other extreme of the adduct
molecules are further connected to give overall 2D layers parallel to (100) (Fig. 4b). The
arrangement originated by the selfꢀaggregation of the doubleꢀtweezer shaped (Hꢀshaped)
adduct molecules, resembles a herringboneꢀtype organization that can be related to the twoꢀ
dimensional closeꢀpacking of hexagonal polygons (Fig. 4c).63 However, along the third
dimension in the crystal structure, [100], infinite π–stacks are not generated and the 2D
layers are packed only through van der Waals interactions and one C−H⋅⋅⋅π contact (vide
infra).
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Figure 4. Representative fragments of the crystal structure of A1. a) Tecton formed between three N→B
adducts by means of π⋅⋅⋅π and C−H⋅⋅⋅O interactions between the bpy linker of the central molecule and the Bꢀ
phenyl rings of two additional adducts; b) 2D layer with herringboneꢀtype pattern formed through π⋅⋅⋅π and
C−H⋅⋅⋅O interactions; c) Schematic representation of the molecular aggregates shown in (b) by ellipsoidal
rings, illustrating the relationship to 2D closeꢀpacking of hexagonal polygons. Note: For clarity, only
hydrogen atoms involved in intermolecular contacts are shown.
The crystal structures of A2−A5 exhibit herringboneꢀtype patterns similar to A1 (Fig. S13).
Nevertheless, only compounds A2−A4 accomplish π⋅⋅⋅π interactions as found in A1 with
the general tendency that the centroid⋅⋅⋅centroid distances increase with increasing number
of fluorine atoms (A1: 3.87 Å; A2: 3.89 Å; A3: 3.83/4.14 Å; A4: 4.00/4.27 Å). At the same
time, the π–stacked aromatic rings adopt a less parallel orientation as indicated by the angle
formed between neighboring mean planes (A1: 15.6°; A2: 15.1°; A3: 31.4/15.2°; A4:
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19.4/35.0°).70 As in A1, the π⋅⋅⋅π interactions are accompanied by a series of C−H⋅⋅⋅O and
C−H⋅⋅⋅π contacts (Fig. S13a−S13c, Table S1). Interestingly, in the crystal structure of A5
π⋅⋅⋅π contacts are absent in favor of a series of C−H⋅⋅⋅O, C−H⋅⋅⋅π and C−H⋅⋅⋅F contacts (Fig.
S13d, Table S1).
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In the third dimension, the packing of the aboveꢀdescribed 2D layers resembles also similar
characteristics in all five crystal structures (A1−A5); however, there are important
differences in the type of intermolecular contacts.71 Comparison of the fragments given in
Figures 5a (A1) and 5b (A2) shows that neighboring layers are connected aside from van
der Waals interactions by C−H⋅⋅⋅π contacts, which in A2 are complemented by doubleꢀ
bridged homodimeric hydrogen bonding motifs that are facilitated through C−H⋅⋅⋅F
contacts between the Bꢀaryl substituents of neighboring molecules (Table S1). The
presence of stabilizing C−H⋅⋅⋅F contacts in A2 is reflected in the increase of the melting
point for this compound when compared to isostructural A1 (A1, 168ꢀ170 °C; A2, 185ꢀ190
°C). In contrast to A2, the crystal structure analysis of isostructural A3 and A4 reveals type
I F···F contacts (θ1=θ2 for C−F···F−C) instead of C−H⋅⋅⋅F interactions, as shown in Figures
5c and 5d. The θ1/θ2 values are 79.1(1) and 86.27(8)o for A3 and A4, respectively, and the
corresponding F⋅⋅⋅F distances are 2.967(2) and 2.761(1) Å. To date, the nature of F⋅⋅⋅F
contacts is still controversial due to the low polarizability of fluorine.72,73 In A3, the F⋅⋅⋅F
distance approaches the sum of the van der Waals (vdW) radii of two fluorine atoms (ΣF,F
=
2.94 Å), but the F⋅⋅⋅C distance of 3.020(3) Å to the ipsoꢀcarbon atom of the neighboring
molecule is slightly smaller than the vdW radii of the atoms involved (ΣC,F = 3.17 Å) with a
C−F⋅⋅⋅C angle of 105.2(1)°. On the contrary, in the trifluorophenyl derivative A4, the F⋅⋅⋅F
distance of 2.761(1) Å is significantly smaller than the sum of the van der Waals radii and
the θ1/θ2 values approach 90°. The analogous F⋅⋅⋅C distance decreases to 2.996(2) Å and the
C−F⋅⋅⋅C angle increases to 113.13(9)°. These data show that the F⋅⋅⋅F contacts might be
complemented by F⋅⋅⋅Cπ contacts, which were previously postulated by Guru Row on the
basis of a CSD study of perfluorinated molecules.73 Similar to A1/A2, the melting point for
the trifluorophenyl derivative A4 is significantly larger than for the isostructural
difluorophenyl analogue A3 (A3, 192ꢀ196 °C; A4, 212ꢀ216 °C), which in this case might
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be attributed to the larger number of short intermolecular C−H⋅⋅⋅O and C−H⋅⋅⋅π contacts
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6
(Table S1).
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9
Figure 5e represents the supramolecular polymeric chain along [010] in the crystal structure
of A5, which is facilitated by homodimeric motifs based on bifurcated C−H⋅⋅⋅F hydrogen
bonds. A second C−H⋅⋅⋅F contact is formed between the second orthoꢀfluorine atom of the
Bꢀaryl group and the second CαꢀH hydrogen of the bpy linker, and the C−H⋅⋅⋅F hydrogen
bonds are accomplished by C−H⋅⋅⋅O and C−H⋅⋅⋅π contacts.
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Figure 5. Perspective views of the crystal structures of A1−A5, showing the connectivity between the 2D
layers described in Figures 4 and S13: a) A1 showing C−H⋅⋅⋅π contacts, b) A2 exhibiting homodimeric
synthons based on C−H···F contacts and a C−H⋅⋅⋅π contact, c) A3 (isostructural with A4) with F···F contacts
(type I) between the BꢀarylF moieties, and d) A5 showing homodimeric motifs with bifurcated C−H···F and
an additional C−H···F contact with the bpy linker.
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3.3. Solidꢀstate inclusion of aromatic guest molecules
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3.3.1 Preparation of inclusion complexes with aromatic hydrocarbon derivatives
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8
9
The crystallographic analysis of A1−A5 revealed that π−stacking constitutes an essential
element in most of the crystal structures. From the vast literature on π⋅⋅⋅π interactions,66ꢀ69 it
is wellꢀknown that π⋅⋅⋅π contacts between electronꢀdeficient and electronꢀrich aromatic
species are particularly strong and frequently accompanied by electron transfer. In order to
analyze the potential of the N→B adducts described herein for complex formation, the
molecular electrostatic potential (ESP) surface map of A3 and a series of aromatic guest
molecules was calculated (i.e. benzene, toluene, oꢀxylene, mꢀxylene, pꢀxylene, mesitylene,
naphthalene, aniline, 1ꢀnaphthol, 3ꢀbromophenol, tetrathiafulvalene and benzonitrile). For
bpy, 24fphbe and the guest molecules first the geometry was optimized by performing
DFT calculations using the B3LYP/6ꢀ31G* level of theory implemented in
SPARTAN’10.74 However, for A3 the surface was mapped using the singleꢀcrystal data,
because the geometry optimization changed the molecular structure significantly. As
expected, the bpy linker in the 2:1 adduct A3 exhibits a ESP surface with positive charge
due to the participation in two N→B bonds, while the catecholate segments are potential
π−donor sites with negative polarization. The BꢀarylF group effectuates an intermediate
potential (Fig. 6). The potential aromatic guest molecules exhibit zones of negative
electrostatic potential in all cases, which with exception of benzonitrile includes the
π−cloud, indicating that the formation of inclusion complexes with A1−A5 is plausible.
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Figure 6. Molecular electrostatic potential surface maps for A3 and a series of potential aromatic guest
molecules (benzene, toluene, oꢀxylene, mꢀxylene, pꢀxylene, mesitylene, naphthalene, aniline, 1ꢀnaphthol, 3ꢀ
bromophenol, tetrathiafulvalene and benzonitrile). Note: Electrostatic potential (ESP) layouts are mapped on
the electron density isosurface
ESP.
ρ
(r) = 0.002 e Åꢀ3. Blue color indicates positive ESP and red color negative
In view of the above analysis and previous finding that 2:1 boronic ester−dipyridyl ligand
adducts are susceptible to inclusion of aromatic guest molecules including polyaromatic
hydrocarbons (PAHs),24 the inclusion properties of a selected N→B adduct were explored.
For this purpose, compound A3 was selected mainly because of two reasons. First, the
fluorine substituents decrease the electron density of the Bꢀaryl group, making it less
competitive for π(host)−π(guest) complex formation. Second, the presence of orthoꢀ
fluorine atoms in the N→B adduct could enable the generation of C−H⋅⋅⋅F contacts with the
aromatic guest molecules (see Fig. 3b).
In the initial phase of the guest inclusion experiments within the solidꢀstate structure of A3,
both aromatic hydrocarbon derivatives containing electronꢀwithdrawing (e.g. nitrobenzene,
benzonitrile, benzaldehyde, benzoic acid, etc.) and electronꢀdonating substituents (e.g.
methylbenzene derivatives, aniline, phenols, etc.) were employed. Nevertheless, mostly
only electronꢀrich systems were susceptible to the formation of inclusion complexes, which
is generally indicated by a color change. Thus, slow evaporation of solutions of 2:1
stoichiometric mixtures of 24fphbe and bpy in aromatic solvents such as benzene, toluene,
oꢀ, mꢀ, pꢀ xylene, mesitylene, benzonitrile and aniline (in this case dissolved in acetonitrile)
gave orange solid crystalline materials (Scheme 3), for which PXRD and SCXRD analysis
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complemented by IR spectroscopic and thermogravimetric studies (Fig. S14ꢀS38,
Supporting Information) revealed the formation of solvates: A3⊃BEN, A3⊃TOL,
A3⊃oXYL, A3⊃mXYL, A3⊃pXYL, A3⊃MES, A3⊃BENCN, A3⊃ANI. Similarly,
when heavier solid aromatic hydrocarbon derivatives (i.e. naphthalene, 1ꢀnaphthol and 3ꢀ
bromophenol) dissolved in acetonitrile were combined with 24fphbe and bpy, guest
inclusion was observed resulting in cocrystals according to the most accepted definition for
this type of solid phase:75 A3⊃NAP, A3⊃NAPOH, A3⊃BrPOH. Slow evaporation of a
mesitylene solution containing 24fphbe, bpy and tetrathiafulvalene yielded the cocrystal
solvate A3⊃TTF⋅MES (Table 4).
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Scheme 3. Aromatic compounds employed for the formation of inclusion complexes with A3.
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Table 4. Composition, method of preparation and crystal structure type of the inclusion compounds derived
4
5
from A3.
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8
Adduct
Guest
Hostꢀguest
ratio
Preparative method
Crystal
structure type
9
Benzene (BEN)
Toluene (TOL)
1:1
1:1
1:1
1:1
1:1
1:1
1:2
1:2
1:1
1:2
1:2
1:2:2
Solvent1
Solvent1
Solvent1
Solvent1
Solvent2
Solution2
Solvent1
Solvent1
Solvent1
Solution2
Solution2
Solution3
I
I
A3⊃BEN
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A3⊃TOL
metaꢀXylene (mXYL)
paraꢀXylene (pXYL)
Aniline (ANI)
I
A3⊃mXYL
A3⊃pXYL
A3⊃ANI
I
I
Naphthalene (NAP)
orthoꢀXylene (oXYL)
Benzonitrile (BENCN)
Mesitylene (MES)
1ꢀNaphthol (NAPOH)
3ꢀBromophenol (BrPOH)
TTF, MES
I
A3⊃NAP
II
III
IV
V
V
VI
A3⊃oXYL
A3⊃BENCN
A3⊃MES
A3⊃NAPOH
A3⊃BrPOH
A3⊃TTF⋅MES
1
Crystallization method 1: Singleꢀcrystals were obtained after slow evaporation of a briefly heated solution
containing the components required for adduct formation (24fphbe and bpy) and the aromatic guest as
solvent.
Crystallization method 2: Singleꢀcrystals were obtained after slow evaporation of a briefly heated mixture
of the aromatic guest and the components required for adduct formation in acetonitrile.
Crystallization method 3: Singleꢀcrystals were obtained after slow evaporation of a briefly heated mixture
of tetrathiafulvalene and the components required for adduct formation in mesitylene.
2
3
The combined PXRD and SCXRD studies enabled to establish that the solid phases derived
from A3 and the selected aromatic hydrocarbon derivatives afforded a total of six crystal
structure types (Tables 4 and 5, Fig. S25). The most frequent crystal structure type I was
observed for A3⊃BEN, A3⊃TOL, A3⊃mXYL, A3⊃pXYL, A3⊃ANI, and A3⊃NAP (for
unit cell parameters see Table 5). For the remaining inclusion complexes, the unit cell
parameters are significantly different from each other (crystal structure types II−VI),
indicating different crystal structures. In addition, the inclusion complexes have varying
hostꢀguest stoichiometries, i.e. 1:1 for A3⊃BEN, A3⊃TOL, A3⊃mXYL, A3⊃pXYL,
A3⊃ANI, A3⊃NAP and A3⊃MES, 1:2 for A3⊃oXYL, A3⊃BENCN, A3⊃NAPOH and
A3⊃BrPOH and 1:2:2 for A3⊃TTF⋅MES, as established by SCXRD analysis and TG
measurements (vide infra).
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Table 5. Selected crystallographic data for the inclusion complexes A3⊃BEN, A3⊃TOL, A3⊃ANI, A3⊃oXYL, A3⊃BENCN,
A3⊃NAPOH, A3⊃BrPOH and A3⊃TTF⋅MES.[a]
8
9
Crystal data[b]
A3⊃
BEN[c]
A3⊃
TOL[c]
A3⊃
ANI[d]
A3⊃
oXYL[c]
A3⊃
BENCN[c]
A3⊃
NAPOH[d]
A3⊃
BrPOH[c]
A3⊃
TTF⋅MES[c]
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C34H22B2F4N2O4,
C6H6
C34H22B2F4N2O4,
C7H8
C34H22B2F4N2O4,
C6H7N
C34H22B2F4N2O4,
2(C8H10)
C34H22B2F4N2O4,
2(C7H5N)
C34H22B2F4N2O4,
2(C10H8O)
C34H22B2F4N2O4,
2(C6H5BrO)
C34H22B2F4N2O4,
2(C6H4S4), 2(C9H12)
Formula
MW (g molꢀ1)
Crystal system
Space group
Radiation source
a (Å)
698.26
Monoclinic
P21/c
712.29
Monoclinic
P21/c
713.28
Monoclinic
P21/c
832.47
Triclinic
Pꢀ1
826.39
Triclinic
Pꢀ1
908.48
Monoclinic
P21/n
966.17
Monoclinic
P21/n
1269.19
Triclinic
Pꢀ1
Mo Kα
7.2038(3)
15.1477(6)
15.1129(6)
90
Mo Kα
7.3534(6)
15.1463(12)
15.1443(19)
90
Cu Kα
Mo Kα
Mo Kα
9.7034(4)
10.0334(5)
23.7112(17)
99.478(5)
92.155(5)
114.320(5)
2060.6(2)
2
Cu Kα
Mo Kα
13.3379(5)
11.4369(3)
14.2468(4)
90
Mo Kα
7.2738(2)
15.1490(3)
15.1105(3)
90
9.9344(5)
10.5481(6)
11.1909(5)
86.855(4)
78.996(4)
62.624(5)
1021.48(10)
1
14.58920(17)
10.15164(12)
14.91846(16)
90
10.6571(6)
11.8609(6)
12.5886(6)
96.084(4)
92.772(4)
109.662(5)
1484.01(13)
1
b (Å)
c (Å)
α (deg)
β (deg)
94.430(4)
90
94.988(9)
90
94.927(2)
90
99.8884(10)
90
108.703(4)
90
γ (deg)
V (Å3)
1644.22(12)
2
1680.3(3)
2
1658.89(7)
2
2176.66(4)
2
2058.50(12)
2
Z
Z’
0.5
0.5
0.5
0.5
2
0.5
0.5
0.5
µ (mmꢀ1)
0.105
0.105
0.895
0.097
0.097
0.838
2.041
0.365
ρ
calcd (g cmꢀ3)
1.410
1.408
1.428
1.353
1.332
1.386
1.559
1.420
R (Fo > 4σFo)
wR2 (all data)
GOF
0.0447
0.1042
1.049
0.0847
0.2396
1.054
0.0548
0.1586
1.048
0.0450
0.1124
1.021
0.0726
0.1856
1.139
0.0446
0.1201
1.043
0.0406
0.0912
1.038
0.0623
0.1795
1.039
CCDC
1574796
1574798
1574802
1574799
1574800
1574803
1574801
1574805
a SCXRD data are available also for A3⊃NAP, but since this derivative is object of a still ongoing research project, herein only the unit cell parameters are included: a = 7.6864(4) Å, b = 14.3332(8) Å, c
= 16.0239(7) Å; = 90.00º, = 91.182(4)º,
= 90.00º, V = 1765.01(15) Å3 (T = 150 K). The unit cell parameters indicate that this compound is isostructural with A3⊃BEN. For crystals of A3⊃mXYL
and A3⊃pXYL, the data sets acquired did not permit a proper refinement of the guest disorder, giving in consequence large Rꢀvalues. However, the unit cell parameters indicate that these compounds are
isostructural with A3⊃BEN. Unit cell parameters for A3⊃mXYL (T = 100 K): a = 7.4361(5) Å, b = 14.7577(8) Å, c = 15.8710(8) Å; = 90.00º, = 92.971(5)º,
= 90.00º, V = 1739.34(17) Å3. Unit cell
= 90.00º, V = 1744.12(4) Å3. b T = 100 K for all compounds except
α
β
γ
α
β
γ
parameters for A3⊃pXYL (T = 100 K): a = 7.70283(11) Å, b = 14.85528(17) Å, c = 15.29263(16) Å;
for A3⊃BENCN, for which data were collected at T = 200 K.
α = 90.00º, β = 94.6582(11)º, γ
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3.3.2 Structural analysis of the 1:1 inclusion compounds A3⊃BEN, A3⊃TOL and
A3⊃ANI
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The isostructural character of the inclusion complexes A3⊃BEN, A3⊃TOL and A3⊃ANI,
is reflected by the similar unit cell dimensions, identical space group (P21/c) and strong
similarities of the molecular conformation and supramolecular arrangement (Tables 5, S2
and S3). Figure 7 shows fragments of the crystalline arrangements of A3⊃BEN, A3⊃TOL
and A3⊃ANI. The molecular structures of the N→B adducts within these crystal structures
are similar between each other and similar to that in A3. The most significant differences
occur for the αpyꢀangle that decreases significantly from 74.9/79.7º to 54.4/66.5/58.6º
(Table S2), reflecting a significant twist of the bpy linker. As illustrated in Figure 7a, in
A3⊃BEN the guest molecules are sandwiched between inclined bpy linkers of two
adjacent adducts, giving infinite π−stacks along [100]. This π−stacking is consistent with
chargeꢀtransfer that gives rise to the orange color of the crystals (A3 is yellow).76 Within
the π−stacked pillars, the benzene molecules are displaced parallel to the pyridyl fragments
and located over the central Cpy−Cpy bond of bpy (centroid⋅⋅⋅centroid = 3.98 Å). The
interplanar angle between the mean planes of bpy and benzene is 6.2°, which is
significantly smaller than the values found for the selfꢀaggregation complex within the
crystal structure of A3 (15.2 and 31.4°, vide supra). In A3⊃BEN, the π−stacking is
complemented by C−H⋅⋅⋅O contacts formed between the C−H hydrogen in 3ꢀposition of the
BꢀarylF group and one of the oxygen atoms (O2) of the catechol ester. In addition, the guest
molecules are stabilized by two bifurcated C−H⋅⋅⋅F contacts with the B−arylF groups from
two different adduct molecules. Neighboring molecules within the π−stacks are related by
the symmetry elements of the P21/c space group resulting in a structural organization, in
which each benzene guest is surrounded by a total of four N→B adducts (Fig. 7b). The
cage formed between the four adduct molecules is stabilized by a total of six C−H⋅⋅⋅O,
C−H⋅⋅⋅π and C−H⋅⋅⋅F contacts involving the cat, py and Bꢀaryl moieties (Table S3), and
resembles the cage formed in the inclusion complex of the dihydroxynaphthalene ester of
phenylboronic acid with 1,2ꢀdi(4ꢀpyridyl)ethylene as diamine linker and anthracene as
guest.24
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Although the essence of the supramolecular organization of A3⊃BEN is conserved in
A3⊃TOL and A3⊃ANI, the orientation and the type of intermolecular contacts of the
guest molecule change somewhat due to the different steric requirements and functional
groups. Within the crystal structure, toluene and aniline are disordered over two positions,
exhibiting C−H⋅⋅⋅F contacts in the first case and in the second case also N−H⋅⋅⋅F contacts
with the amino group (Fig. 7c, Table S3).
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Figure 7. Fragments of the isostructural crystal structures of A3⊃BEN, A3⊃TOL and A3⊃ANI. a)
Perspective view of the infinite π−stacks in A3⊃BEN along [100] formed between the N→B adducts and the
benzene guest molecules. b) Illustration of the guest cavity formed between four adjacent adduct molecules
having approximate mutual perpendicular orientation. c) Additional stabilization of the guest molecules in
A3⊃TOL and A3⊃ANI by C−H⋅⋅⋅F and N−H⋅⋅⋅F interactions, respectively. Note: In all crystal structures, the
Bꢀaryl group exhibits positional disorder giving orientations with opposite directions of the orthoꢀfluorine
atoms. In A3⊃TOL and A3⊃ANI, the guest molecules and in A3⊃TOL additionally the bpy units are
disordered. For clarity, only the most abundant orientation is shown.
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3.3.3 Structural analysis of the 1:2 and 1:2:2 inclusion compounds A3⊃oXYL,
A3⊃BENCN, A3⊃NAPOH, A3⊃BrPOH and A3⊃TTF⋅MES
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The 1:2 and 1:2:2 inclusion complexes formed with oꢀxylene (oXYL), benzonitrile
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(BENCN),
1ꢀnaphthol
(NAPOH),
3ꢀbromophenol
(BrPOH)
and
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tetrathiafulvalene/mesitylene (TTF/MES) comprise as common characteristics the presence
of 1:2 π−π bonded sandwichꢀtype complexes formed between the N→B adduct and the
guest, which in the case of A3⊃TTF⋅MES is TTF (Table 5). Although in all compounds
the guest molecules are located above and below the bpy linker, there are significant
differences in the structural arrangement of the guest−adduct−guest triades, which can be
illustrated on hand of the geometric parameters listed in Tables S4 and S5.
Figure 8 provides perspectives of the 1:2 π−π stacked complexes extracted from the crystal
structures of A3⊃oXYL, A3⊃BENCN, A3⊃NAPOH, A3⊃BrPOH and A3⊃TTF⋅MES.
For comparison, the analogous fragment of the infinite π−π stack found in the 1:1 inclusion
complex A3⊃BEN is also included. Visual comparison and consideration of the geometric
parameters of the molecular structures show that the guest inclusion and concomitant π⋅⋅⋅π
interaction varies from case to case, introducing significant changes in the molecular
structures, which ultimately affect the overall supramolecular organization (Tables S4 and
S5). In terms of π⋅⋅⋅π interactions, benzene, oꢀxylene, 1ꢀnaphtol, 3ꢀbromophenol and
tetrathiafulvalene establish parallel displaced geometries with centroid(guest)⋅⋅⋅bpy(mean
plane) distances in the range of 3.41−3.56 Å, while benzonitrile exhibits an approximate
faceꢀtoꢀface orientation with the −CN group being almost parallel to the central Cpy−Cpy
bond. In the faceꢀtoꢀface arrangement with the electronꢀdeficient benzonitrile, the
centroid(guest)⋅⋅⋅bpy(mean plane) distance is significantly larger (3.76−3.78 Å; Table S5).
An important difference between A3⊃BEN and A3⊃oXYL is the direction of the
displacement, which in the case of A3⊃BEN occurs towards the center of the bpy linker,
but towards a single pyridyl ring for each oXYL unit in A3⊃oXYL. A similar arrangement
as in A3⊃oXYL is found in A3⊃BrPOH. However, the guest molecules composed of two
aromatic rings (viz. 1ꢀnaphthol in A3⊃NAPOH and tetrathiafulvalene in A3⊃TTF⋅MES)
interact with both pyridyl groups. In A3⊃TTF⋅MES, the TTF molecules are curved (Fig.
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Crystal Growth & Design
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2
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8f), which is a common phenomenon77,78 and can be attributed to the larger atom size of
sulphur when compared to carbon, nitrogen and oxygen.
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As seen from Figure 8, the N→B adducts are able to adapt to the steric requirements of the
different guest molecules. With BrPOH, the catecholate and B−aryl moieties in the tweezer
structure get closer, whilst with BENCN and TTF the cavity is widened as indicated by the
values of the βpy,cat and βpy,aryl angles (A3⊃BrPOH, 100.1 and 99.8°; A3⊃BENCN,
122.8/124.2° and 107.6/105.6°; A3⊃TTF⋅MES, 110.3 and 110.4°) and the
centroid(cat)⋅⋅⋅centroid(aryl) distances (A3⊃BrOH, 11.45 Å; A3⊃BENCN, 13.03/12.95 Å;
A3⊃TTF⋅MES, 12.59 Å). For the remaining compounds, intermediate values are noted
(Table S4). In addition, with NAPOH, BrPOH and TTF the B−aryl mean planes approach
an orthogonal orientation with respect to the NBCi plane (A3⊃NaPOH, 69.2°;
A3⊃BrPOH, 66.6°; A3⊃TTF⋅MES, 75.0°), thus enabling C−Hguest⋅⋅⋅πhost or Brguest⋅⋅⋅πhost
contacts with both extremes of the adduct (Fig. 8dꢀ8f). On the contrary, in the presence of
oXYL and BENCN C−H⋅⋅⋅π contacts are only formed with the catecholate group. In the
latter, the Bꢀaryl group is almost parallel to the NBCi reference plane (A3⊃oXYL, 5.1°;
A3⊃BENCN, 0.7 and 2.8°) to accomplish a bifurcated C−H⋅⋅⋅F interaction in A3⊃oXYL
(Fig. 8b) and a C−H⋅⋅⋅N interaction in A3⊃BENCN (Fig. 8c). The different orientations of
the fluorinated aryl rings have important consequences for the supramolecular organization
in the three dimensional space, particularly because the ortho−fluorine atoms can
participate in intermolecular C−H⋅⋅⋅F and F⋅⋅⋅F contacts (vide infra).
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